How to create geometry with "point and click" and set number of vertices?
This is a rather trivial problem. In edit mode, in the 3D viewport, locate the 3D cursor where you want the first vertex (starting point) and while holding down the CTRL key, click the left mouse button. Then, move the cursor to the location at which you want the other vertex (end point), hold down the CTRL key, while clicking the left mouse button again. Now, select the first vertex, and while holding the SHFTkey, click the vertex that you chose as the start point, and press the Fkey to join the two vertices with a line segment. With the line segment selected, use the subdivide tool (press the spacebar, and enter "subdivide" in the search bar) to create the number of intermediate vertices desired. The default behavior of the subdivide tool us to space the intervenient vertices equally between the end points.